The present invention relates to a fuel cell anode for the oxidation of methanol, which contains at least one platinum metal or alloys of platinum metals as the primary, catalytically active component.
The direct anodic oxidation of methanol, which is especially important for the use of membrane fuel cells as sources of power for automobiles, requires uneconomically large loadings of noble metals in the anode, even when the direct methanol cell is operated at a greatly increased temperature; namely, at 130.degree. C. [W. Preidel, K. Mund, K. Pantel, G. Starbeck, G. Luft, M. Waidhas, VDI report 1174 211 (1995)]. The state of the prior art teaches the use of an alloy catalyst on a carbon black support which contains platinum and ruthenium as cocatalyst in the molar ratio of 1:1 [H. -F. Oetjen, V. Schmidt, U. Stimming, F. Trila, J. Electrochem. Soc. 143, 3 83 8 (1996)].
The platinum component on the surface of the alloy crystallites, which are dispersed on a nano-scale on the carbon black, is used substantially for chemisorptively caused cleavage of the C-H bond in the methanol molecule whose oxidative degradation ultimately leads to the production of carbon monoxide, or a species which is closely related to carbon monoxide, and poisons the platinum as a result of strong absorptive bonding thereto [S. Wilhelm, T. Iwasita, W. Vielstich, J. Electroanal. Chem. 238, 383-391 (1987)].
Ruthenium forms surface oxides by anodic oxidation significantly more readily than does platinum (at about +500 mV against a reversible hydrogen electrode (RHE=Reversible Hydrogen Electrode), compared with +800 mV against a RHE for platinum) and is capable of transferring the oxygen in the surface oxide to carbon monoxide which is very mobile on the surface of the catalyst [K. A. Friedrich, K. -P. Geyzers, U. Linke, U. Stimming, J. Stumper, J. Electroanal. Chem. 402, 123-128 (1996)]. This is then oxidized to only very weakly absorbed carbon dioxide and thereby partly counteracts the poisoning of the catalyst caused by CO absorption.
The disadvantage of using ruthenium as a cocatalyst is the fact that, at temperatures of 80 to 100.degree. C., the conventional range for the operating temperature of pressureless operating membrane fuel cells, it is not possible for the open-circuit potential of the methanol anode to get within less than +300 mV of the equilibrium potential of methanol oxidation, due to the still relatively too high equilibrium potential of ruthenium oxide formation, which is why cell operation threatens to become uneconomical due to too high a loss of power. In addition, the use of a second noble metal from the group of platinum metals as a cocatalyst is too expensive due to the high stoichiometric ratio required for use in fuel cells.
It has been known for some time, from fundamental investigations, e.g., by Sandstede [G. Sandstede ed., from Electrocatalysis to fuel cells, University of Washington Press, Seattle, London, 1971] and more recently by Tseung [P. K. Shen, A. C. C. Tseung, J. Electrochem. Soc. 141, 3082 (1994)] that non-platinum metals are also suitable as cocatalysts for the anodic oxidation of methanol catalyzed by platinum. These findings have also been demonstrated with catalysts on carbon black or activated carbon supports in membrane fuel cells [M. Gotz, H. Wendt, in Gesellschaft deutscher Chemiker Monographie vol. 10, ed. by F. J. Kruger, published 1988]. It has been shown, however, that a cocatalytic effect which is comparable to that of ruthenium, even using the cocatalytically most active elements, in particular tungsten and molybdenum for methanol and tin for anodic CO oxidation, cannot be achieved.
Therefore, an object of the present invention is to provide a fuel cell anode using a cocatalyst for the electrocatalytic oxidation of methanol in acid electrolytes, in particular in acid ionomer electrolytes such as NAFION.RTM., which produces comparable or better performance data than a fuel cell anode with a Pt/Ru catalyst and which is resistant to poisoning of the platinum.